[sane-devel] brightness/contrast/gamma LUT ideas?

[m. allan noah]

On Thu, 20 Jul 2006, Rene Rebe wrote:
> On Tuesday 18 July 2006 19:35, m. allan noah wrote:
>>> As far as I know, xsane automatically calculates and downloads a gamma
>>> table for brightness / contrast / gamma if the backend advertises it.
>>
>> interesting! do you know of a backend that does this? i would like to see
>> the option structures which are used... of course, not everyone uses
>> xsane, so i might have to steal the code from it :)
>
> I think xsane does it for any backend with gamma tables.

as defined by having an option named by the macro SANE_NAME_GAMMA_VECTOR?

> The Avision backend has gamma tables as well as emulation. The calculation
> of brightness/contrast was pasted from an old gimp version and might have
> the undesired, imperfect effects as you outlined in your previous mail.
>
> Watch for brightness_contrast_func and send_gamma in the avision.c.
>

yes, after looking at your implementation and doing quite a bit of 
reading, it appears that my previous mail may have been incorrect in its 
assertion of those defects. it would seem that brightness/contrast as 
normally used have relationship that varies quite a bit depending on the 
author of the software package in question :)

i have played a bit with the xsane histogram, and watched its affect on 
the brightness/contrast/gamma sliders.

it would seem that really more than 3 controls are needed, and saneopts.h 
agrees, listing option names for highlight, shadow, white point, and 
black point, in addition to gamma. several 'levels' type controls i found 
documented on the web use those 5 options instead of 3 separate controls
(brightness/contrast/gamma)

it seems, looking thru several backends, that there are quite a few 
backends that use some form of gamma table internally, and could perhaps 
benefit from some code sharing at the sanei level, as well as the xsane 
level.

i need to do more research.

allan

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